With the advances in MEMS technology it is possible to fabricate small size and high performance electromechanical energy harvesters converting ambient vibration to electrical potential using MEMS fabrication techniques [1]. Electromagnetic, electrostatic, and piezoelectric transduction mechanisms are the most popular approaches for vibration-based energy harvesting [2]. Among these transduction mechanisms piezoelectric energy harvesting is more attractive due to high voltage output and no need of separate voltage source to initiate the conversion process as in the electrostatic converters [3].
MEMS technology has recently become crucial for biomedical implants as it enables the implementation of smart devices with features that range in size from millimeters to sub-micrometers [4]. Especially in the last two decades, integration of MEMS technology with biomedical industry attracted the attention of many researchers [5]. One of the most promising applications of MEMS technology for biomedical applications is hearing aids.
Hearing impairment is a common disease affecting the patient's quality of life by limiting the social interaction of him/her with the environment. Several types of diseases and various solution methods have been proposed in the past. Being one of the most popular solutions, cochlear implants provide effective and aesthetic solutions for patients suffering hearing impairment. Cochlear implants have three main components in common: a microphone, a signal processor, and an electrode [6]. The microphone converts the incoming sound waves to electrical signals. The signal processor calibrates the amplitude and frequency of these signals, and then transfers them to the electrode, where the corresponding auditory nerves are stimulated.
Microphone is the most critical component of cochlear implants since it converts the acoustic pressure waves into electrical signals. However since the microphone is mounted outside the body it prevents patients' continuous access to sound (while swimming, showering etc.), and constitutes a potential for hardware damage and decreases cosmetic appeal [9].
Up to now various devices to replace the microphone component of cochlear implants which is mounted outside of the body and to reduce the battery need of cochlear implants have been reported.
U.S. Pub. No. 20030012390 reports a vibration detector device suitable for use instead of a microphone in cochlear implants. This device incorporates resonator bars with varying resonation frequencies due to varying thicknesses. Although the microphone used in conventional cochlear implants which is mounted outside the body can be reduced by using the suggested device, this device cannot eliminate the need for battery.
U.S. Pat. No. 6,264,603 describes a vibration detector for sensing the vibration amplitude and direction. The reported device is not capable of generating energy for reducing the battery need of cochlear implants.
US. Pub. No. 20050113633 describes an electromechanical converter converting vibration of ossicles to electrical signals. It is reported that the use of thin elliptic piezoelectric element reduces the power consumption of the device. However, since a single elliptic thin element is used for detecting vibrations, it is not possible to make use of resonance phenomena for increasing the voltage output of piezoelectric element. Therefore stimulation of the auditory nerves without processing the generated signals with electronic unit is not possible.
U.S. Pat. No. 6,261,224 describes a piezoelectric structure coupled to an auditory element such as malleus to be used as both an actuator and a sensor. It is noted that the device is capable of generating a potential due to vibrations of the auditory elements; however the generated voltage is processed with the electronic unit of the implantable system. The generated voltage is used for detecting the frequency of the vibrations and dissipated at the signal processing step. Therefore, again the reported device is not capable of reducing the battery need of conventional cochlear implants.
Another device to be used as a frequency detector is disclosed in U.S. Pat. No. 5,856,722. In this document, a microelectromechanical system is proposed. However this device again does not offer a solution to the power consumption problem of implants.
The need of a battery in cochlear implants is a problematic issue. Patients have to carry a battery pack and recharge them periodically to power up the cochlear implant system. U.S. Pat. No. 3,456,134 describes a piezoelectric energy converter for electronic implants. The device is aimed to convert vibrations due to body motions into electrical energy for driving the implants. However, the suggested device is not capable of converting the acoustical pressure waves or vibrations of ossicular chain to meaningful potential output. Therefore this device cannot be used as a microphone to sense the frequency of acoustical sound pressure waves.
Recently high performance energy harvester devices are developed by using piezoelectric principle. Lee et al. fabricate an aerosol deposited PZT micro cantilever beam operated at 214 Hz gives up to 4,127 V output voltage, which shows that MEMS piezoelectric energy scavenger can be used for both sensing the frequency and generating potential required for 250 Hz-4000 Hz frequency range [7].
Based on the present state of art, it is therefore an object to provide a totally implantable device, capable of detecting the frequency of the acoustic sound pressure waves vibrating the tympanic membrane and reducing the battery need of the cochlear implant device, and that provides long-term stability and biocompatibility.
The object is solved with a totally implantable device mounted to ossicular chain, having multiple cantilever beams with predetermined natural frequencies to sense the incoming vibrations and generate required voltage by making use of resonance phenomena to stimulate the auditory nerves.